Electroplating method and apparatus for electroplating high aspect ratio thru-holes

ABSTRACT

Electroplating apparatus and methods for plating high aspect ratio thru-holes in printed circuit boards and the like. For copper plating thru-holes in printed circuit boards, the printed circuit board is immersed in a tank of electrolyte. Surrounding both sides of the printed circuit board are a plurality of cell-like structures which contain mechanical electrolyte agitating members, filters and electrolyte ion replenishment anode baskets. In addition, a pump is provided to encourage exhausting of the electrolyte from local regions of each side of the printed circuit board to encourage a general but definite flow of electrolyte through any thru-holes in the printed circuit board. The general flow of electrolyte as encouraged by the pump is through the ion replenishment anode basket to the mechanical agitator adjacent the printed circuit board, through any thru-holes in that region of the printed circuit board and to some extent around the printed circuit board, past the mechanical agitator on the other side of the printed circuit board and through a filter adjacent the exhaust manifold for recirculation back and forth through similar cells disposed along the length of the circuit board. The mechanical agitation avoids ion depletion in the electrolyte adjacent the printed circuit board and particularly adjacent the edges of and within the thru-holes by assuring fast and continuous interchange of the &#34;surface&#34; electrolyte with the bulk electrolyte, which itself is replenished at an adequate rate by the background flow established by the pump. Alternate methods and apparatus are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of The Invention.

The present invention relates to electroplating apparatus and methods,and more particularly the methods and apparatus for the plating ofprinted circuit board thru-holes and the like.

2. Prior Art.

For various reasons, the proper plating of thru-holes in printed circuitboards is an important aspect of the fabrication of certain types ofprinted circuits. In particular, far superior mechanical and electricalconnection may be made to electronic component leads if the printedcircuit board through holes are suitably copper plated, as the solderedbond to the component leads will in general run the entire length of thethru-holes rather than merely the length of whatever solder maniscusthere is on the soldered component lead and the face of the printedcircuit board on which soldering takes place. Further, in multilayerprinted circuit boards, plated through holes are used to make electricalcontact with buried printed circuit layers, so that a high qualityplating and appropriate copper buildup throughout the entire length ofthe thru-hole is critical to the accomplishment of high reliabilitymultilayer interconnects. The achievement of such thru-hole platinghowever is difficult for various reasons. In particular in aconventional plating tank the edges of the mouth of a through hole on aprinted circuit board cause particularly high currents and plating ratesat that location, effectively depleting the ions in the electrolytepassing into the thru-holes so that the plating rate and plating qualitygrossly diminish along the depth of the thru-holes depending upon theaspect ratio, that is, the length to diameter ratio of the thru-holes.Consequently, high aspect ratio thru-holes cannot be repeatably andreliably plated using conventional plating methods.

In the prior U.S. Pat. No. 4,174,261, there is disclosed methods andapparatus for providing high and uniform processing rates forelectroplating, deplating, etching and the like, substantiallyindependent of the surface geometries of the article subjected to theprocess. In accordance with that process in an electroplatingapplication, the article to be plated is supported on a cathode so thatthe electrolyte may be forceably sprayed on the article from an array ofspray nozzles adjacent the surface thereof. Intermixed with the array ofspray nozzles may be a second array of openings providing suction tolocally remove most of the sprayed electrolyte after impingement on theworkpiece. The net effect is that fresh electrolyte is constantly beingsprayed onto the article being plated, including being sprayed intothru-holes of a printed circuit board, with the spent electrolyte beingquickly removed from the workpiece before it has an opportunity toshield the surface thereof from the spray of fresh electrolyte.Functionally, the method and apparatus of that patent work very well,resulting in uniform, very high density plating on flat surfaces, andvery good deposition distribution along relatively deep thru-holes inprinted circuit boards, not generally achievable with other techniques.The process is less than optimum however from a noise and energystandpoint, as a relatively high pumping power is required to give thebest results, and the process itself is relatively noisy, particularlywith multiple tanks operating in a typical production environment.

BRIEF SUMMARY OF THE INVENTION

Electroplating apparatus and methods for plating high aspect ratiothru-holes in printed circuit boards and the like. For copper platingthru-holes in printed circuit boards, the printed circuit board isimmersed in a tank of electrolyte. Surrounding both sides of the printedcircuit board are a plurality of cell-like structures which containmechanical electrolyte agitating members, filters and electrolyte ionreplenishment anode baskets. In addition, a pump is provided toencourage exhausting of the electrolyte from local regions of each sideof the printed circuit board to encourage a general but definite flow ofelectrolyte through any thru-holes in the printed circuit board. Thegeneral flow of electrolyte as encouraged by the pump is through the ionreplenishment anode basket to the mechanical agitator adjacent theprinted circuit board, through any thru-holes in that region of theprinted circuit board and to some extent around the printed circuitboard, past the mechanical agitator on the other side of the printedcircuit board and through a filter adjacent the exhaust manifold forrecirculation back and forth through similar cells disposed along thelength of the circuit board. The mechanical agitation avoids iondepletion in the electrolyte adjacent the printed circuit board andparticularly adjacent the edges of and within the thru-holes by assuringfast and continuous interchange of the "surface" electrolyte with thebulk electrolyte, which itself is replenished at an adequate rate by thebackground flow established by the pump. Alternate methods and apparatusare also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of the plating apparatus of the present inventionwith the covers over the various cells partially cut away to provide atop view of the internal structure thereof.

FIG. 2 is a cross section taken along line 2--2 of FIG. 1.

FIG. 3 is a cross section taken along line 3--3 of FIG. 1.

FIG. 4 is a cross section taken along line 4--4 of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

First referring to FIGS. 1, 2, 3 and 4, a top view with part of thecovers cut away and various cross sections through the electroplatingapparatus of the preferred embodiment of the present invention may beseen. An outer tank 20 provides the main containment for the electrolyte22, the tank 20 being generally open at the top except for some form ofdust cover one may choose to use therewith, not shown. Mounted withintank 20 is an inner tank-like structure generally indicated by thenumeral 24. The inner tank-like structure is comprised of a bottom 26,sidewalls 28 and end walls 30 and 32. Fastened to the bottom wall 26 isa channel-like member 34, preferably fabricated from some suitableplastic such as Delrin or the like. A similar slot 36 is positioned nearthe upper portion of the inner tank 24, being retained in position byengagement with transverse walls 38 at an elevation which will allowsome flow of electrolyte over the top of the channel member.

The printed circuit board 40 to be plated is fastened within a smallframe-like rack 42 so that the printed circuit board and frame-like rackmay readily slide as a unit in a longitudinal direction along channelmembers 34 and 36, electrical contact being made to the printed circuitboard through lead 37. A gear motor 44 is provided adjacent the end ofouter tank 20 at an elevation above the electrolyte level to drive asprocket 46 in rotation, which in turn will cause reciprocation of theprinted circuit board and frame-like rack through the crank pin 48 andconnecting rod 50. The reciprocating drive for the printed circuit boardis shown entirely above the electrolyte level, though if desired,particularly depending upon the proportions and size of the printedcircuit board, the crank pin and connecting rod drive may readily belocated midway within the tank, or a second drive may be locatedadjacent the bottom of the tank, as plastic members may readily be usedfor this drive mechanism to avoid any plating thereon when locatedbeneath the electrolyte level.

As may be seen in FIG. 1, the inner tank structure 24 is divided up intoa plurality of substantially square cell regions by the plurality oftransverse walls 38 and longitudinal walls 52. The transverse walls 38are preferably solid walls, that is, substantially impervious toelectrolyte, though the longitudinal walls 52, as may be seen in FIG. 2,are generally provided with a predetermined porosity to allow therelatively uniform flow of electrolyte therethrough. In that regard, thelongitudinal walls 52 could be eliminated if desired, though theirpresence is preferred as they provide an improved flow control, as shallsubsequently be described in greater detail.

Located within each of the inner cells 54 is a paddle wheel-likeassembly 56, being supported on simple journal bearings 58 at the topand bottom thereof, with the center shaft 60 thereon extending above theelectrolyte level and each having sprockets 62 at the top end thereof.These paddle wheels also are preferably fabricated using a materialwhich will not be active in the plating process such as, by way ofexample, a plastic or nonreactive metal, such as stainless steel, monelor titanium. Extending around sprocket 46, idler sprockets 64 andsprockets 62 is a bicycle chain 66, so that the drive motor 44 causingthe reciprocating motion of the printed circuit board also causesrotation of the paddle-like assemblies 56 in each of the inner cells tocause the electrolyte therein to have substantial agitation. This hasthe affect of avoiding ion depletion in the layers of electrolyteadjacent the printed circuit board surface by causing substantial andrelatively immediate interchange between the bulk electrolyte in theseinner cells and the otherwise relatively stagnant surface electrolyteadjacent the surface of the of the printed circuit board. Thus, one ofthe objectives of the present invention is achieved by the agitation ofthe paddle-like assemblies, specifically the substantially avoidance ofion depletion in the electrolyte adjacent the printed circuit boardbeing plated. Obviously, while the extent of agitation adjacent thesurface of the printed circuit board may vary somewhat along the lengthof the printed circuit board, the simultaneous reciprocation of theprinted circuit board assures that no part of the circuit board spendsany significant time in sheltered or slow moving electrolyte.

In addition to the agitation of the electrolyte provided by the paddleassemblies 56, a general flow direction for the electrolyte isestablished by electrolyte pump 68, being supplied with electrolytethrough lines 70 connected to the inner tank structure 24 and deliveringelectrolyte to the inner tank structure through lines 72. As may beperhaps best seen in FIGS. 1 and 2, the electrolyte is first deliveredto cells 74 which function as ion replenishment cells. These cellscontain a porous basket-like structure 76 (see particularly FIGS. 3 and4) within which are placed pieces of copper 84 for replenishment of thecopper ions in the plating solution. The baskets 76 in the preferredembodiment are fabricated from an electrically conductive material whichis substantially inactive to the electrical process such as, by way ofexample, monel, stainless steel or titanium, as mentioned before. As analternative, the baskets may be fabricated from plastic so long as theycontain some means of making electrical contact to the pieces of copper84 therein, such as by way of a conductive rod extending down into thebasket. The top of the baskets are closed by members 78, with electricalconnection being made thereto through electrodes 80 connected tohorizontal bus bars 82 located well above the electrolyte level. Thecopper in the baskets thus serves as the anode for the electroplatingprocess, with pump 68 first delivering electrolyte through these cellsfor ion replenishment.

The electrolyte then proceeds through the adjacent wall 52 (seeparticularly FIGS. 2 and 3) passing into the regions of agitated flowcaused by the rotation of the paddle assembly 56 therein. In thatregard, wall 52 is provided with a predetermined porosity to somewhatrestrict the flow therethrough, so that the flow is relatively uniformlydistributed across the entire area of the wall, rather than perhapsbeing more concentrated adjacent the inlet 72. The paddle assembliesfurther generally rotate with sufficient velocity to break up the flowquite well, so that the flow in the region of the paddle assembly andadjacent the printed circuit board 40 is quite turbulent, providing arapid electrolyte exchange between the electrolyte adjacent the surfaceof the printed circuit board and the bulk electrolyte in the paddlecells.

The paddle cells 54 are also generally closed at the top thereof byclosure members 88, so that the flow encouraged by pump 68 is a flowtoward the face of the printed circuit board. This causes theelectrolyte to also have a general flow through the thru-holes in theprinted circuit board, assuring a constant flow of fresh electrolytetherethrough. Obviously the volume of flow through the printed circuitholes will of course depend upon the number and size of the holes in theprinted circuit board. However, sufficient room for electrolyte flow isprovided around the ends and over the top of the printed circuit boardto control (limit) the pressure buildup on the face of the printedcircuit board, so that the flow through the holes in the printed circuitboard will be relatively uniform independent of the particular numberand size of the holes in that circuit board region.

The flow through the holes in the printed circuit board and around theedges of the printed circuit board proceeds into the turbulent region ofthe paddle cell on the opposite side of the printed circuit board, sothat the flow on the "back" side of the printed circuit board isconstantly broken up and made turbulent. Actually, in the preferredembodiment, the "back" of the printed circuit board has no real suchidentity because of the extent of oscillation of the printed circuitboard in the tank. In particular, as may be best seen in FIG. 1, flowthrough adjacent cells along the length of the board is in oppositedirections. Consequently, oscillation of the board so that all parts ofthe board oscillate between at least two adjacent cells will cause allsurfaces of the board to spend substantially equal time in the regionsof flow of each of the two directions. In this manner no region of theboard is given, on an average basis, any preferred or shelteredpositions, so that plating is very uniform across both surfaces of theboard.

Finally, the flow through the paddle cell on the "back" of the boardproceeds through the adjacent wall 52 to filter cells 90, each of whichcontain a simple rectangular filter strip 92 for removing particulatematter from the electrolyte prior to the electrolyte's return to thepump 68. These filter cells also are closed at the top so that flow isencouraged through the inner tank structure as described, not only bythe increased pressure at the delivery side of pump 68, but also by thereduced pressure at the inlet thereof which, in effect, becomescommunicated to the back side of the board. Obviously, while filteringis preferable to maintain the electrolyte free of contaminatingparticulate matter, other types of filtering may readily be used, suchas line filters in either lines 70 or 72. The filter cells described arepreferred however, as they provide a large filter area, may be easilyvisually inspected and very easily cleaned and/or replaced as desired.

One of the big advantages of the present invention is its ability to puta high quality copper plating throughout the depth of thru-holes in aprinted circuit board of substantially any thickness desired. Inparticular, prior art plating techniques are generally not well suitedfor plating of thru-holes, particularly high aspect ratio thru-holes(thru-holes having a substantial length to diameter ratio). Prior artplating techniques are generally dependent upon the diffusion of ionsthrough relatively stagnant electrolyte. Because of the sharp edge atthe mouth of a thru-hole, the electric field adjacent the mouth of thethru-hole is substantially higher than in other regions on the face ofthe printed circuit board or within the thru-hole. As a consequence, therate of copper build up at the mouth of the thru-hole tends to begreater than in other regions of the printed circuit board. Further,there is very little plating build up and a relatively low qualityplating results deeper within the thru-hole, partially because of somestagnation of the electrolyte in that region and more particularly,because of the depletion of the plating ions from the electrolyte at themouth of the thru-hole as the electrolyte passes through the higherelectric fields in those regions. As a result, a high plating rate andthick plating would occur at the mouth of the thru-holes, with theplating well within the thru-holes being particularly thin andfrequently of very low quality. Obviously in the case of multilayerboards, the failure of the plating to make positive electrical contactwith the intermediate printed circuit board layer may well result in themalfunction or erratic function of an entire system. Adequate plating ofhigh aspect ratio thru-holes in accordance with prior art techniques wasmost difficult indeed.

In comparison to prior art techniques, the present invention has beenfound to provide a very uniform, high quality plating throughout thedepth of high aspect ratio thru-holes with ease. In fact, depending uponthe degree of agitation used, flow rates used, etc., it has been foundwith the present invention that great rates of plating build up deepwithin thru-holes may be achieved than on the face of a printed circuitboard. Cross sections of experimental plated thru-holes, plated inaccordance with the methods and apparatus of the present invention, haveshown substantially uniform plating thicknesses throughout the depth ofthe thru-holes with no meaningful increase in plating thickness at themouth of the thru-hole. It is believed that the maintenance of asubstantial flow through the thru-holes during the plating processprevents any meaningful depletion of the electrolyte at the mouth of thethru-holes as the electrolyte passes therethrough. Further, the effectsof the higher electric field at the mouth of the thru-holes appeared tobe negated by the present invention because of the substantial absenceof the higher plating rates in this area than deeper within thethru-holes. It is believed that this occurs when using the presentinvention because the agitation and flow of the electrolyte makes iondiffusion under the influence of the electric field only relevant in thevery thin film of electrolyte wetting the actual surfaces of the printedcircuit board and thru-holes, and the electroplating has sufficientbuild up so that the sharp mouth regions of the thru-holes are quicklyrounded in comparison to this very thin film thickness, to quicklyeffectively eliminate the higher electric field in that very thin filmof electrolyte.

There has been described herein new and unique methods and apparatus forplating printed circuit boards, and particularly thru-holes thereinwhich allows the rapid plating of the thru-holes with a high qualityplating of substantially uniform thickness throughout the entire lengthof the through holes, even for thru-holes of very high aspect ratio.While the invention has been disclosed and described herein with respectto a preferred embodiment of apparatus for practicing the method, itwill be understood by those skilled in the art that various changes inform and detail may be made therein without departing from the spiritand scope of the invention. By way of example, the paddle-like agitatorsmay take other forms, such as oscillating agitators, reciprocatingagitators and the like. Similarly, the agitators may contain the metalto be plated in internal baskets (proper electrical contact being madethereto) so that separate cells need not be provided therefor. These andother changes do not effect the basic inventive concepts of theinvention, and will be obvious to those skilled in the art.

We claim:
 1. Plating apparatus for the plating of through holes in aprinted circuit board comprisinga container for confining a platingsolution; printed circuit board support means for supporting a printedcircuit board within said container, said support means includingreciprocating means for repetitive movement of said circuit board backand forth in a plane substantially coincident with the plane of thecircuit board; divider means adjacent each side of said printed circuitboard support means for alternatively defining first and second cellsalong each side of said printed circuit board support means; agitatormeans within each of said cells for mechanically providing substantialagitation to a plating solution therein; and pump means for deliveringplating solution to said first cells and for removing plating solutionfrom said second cells, each of said second cells being substantiallyopposite one of said first cells, whereby first plating solution will beencouraged to flow from said first cells through the through holes in aprinted circuit board on said printed circuit board support means andinto said second cells to provide fresh plating solution to the innersurfaces of through holes.
 2. The plating apparatus of claim 1 whereinsaid reciprocating means provides sufficient movement to a printedcircuit board in said printed circuit board support means torepetitively bring each area of each surface of the printed circuitboard containing through holes into alignment with both first and secondcells, whereby flow of plating solution through the through holes on theprinted circuit board will repetitively reverse direction.
 3. Theplating apparatus of claim 1 wherein said agitation means comprisesrotating paddle wheels in each of said cells.
 4. The plating apparatusof claim 1 for electroplating of through holes in a printed circuitboard further comprisingion replenishment means containing at least onepiece of the metal to be plated within the plating solution forreplenishing the ion content in the plating solution in said firstcells; first electrode means for providing electrical contact between afirst terminal of a power supply and said at least one piece of metal tobe plated; and second electrode means for providing electrical contactbetween a second terminal of a power supply and the printed circuitboard to be plated.
 5. The plating apparatus of claim 4 wherein said ionreplenishment means comprises baskets containing pieces of the metal tobe plated in third cells coupled to said first cells and through whichplating solution passes in flowing to said first cells.
 6. The platingapparatus of claim 5 further comprised of fourth cells coupled to saidsecond cells and through which plating solution passes in flowing fromsaid second cells to said pump means, said fourth cells containingfilter means for filtering of plating solution flowing therethrough. 7.Plating apparatus for the plating of through holes in a printed circuitboard comprisinga container for confining a plating solution; printedcircuit board support means for supporting a printed circuit boardwithin said container, said support means including reciprocating meansfor repetitive movement of said circuit board held in said support meansback and forth in a plane substantially coincident with the plane ofsaid circuit board; divider means adjacent each side of said printedcircuit board support means for defining first and second cells alongeach side of said printed circuit board support means; agitator meanswithin each of said first cells for mechanically providing substantialagitation to a plating solution therein, and pump means for deliveringplating solution to said first cells and for removing plating solutionfrom said second cells, said second cells each being substantiallyopposite one of said first cells, whereby fresh plating solution will beencouraged to flow from said first cells through the through holes in aprinted circuit board on said printed circuit board support means andinto said second cells to provide fresh plating solution to innersurfaces of the through holes.
 8. The plating apparatus of claim 7wherein said reciprocating means provides sufficient movement to aprinted circuit board in said printed circuit board support means torepetitively bring each area of each surface of the printed circuitboard containing through holes into alignment with both first and secondcells, whereby flow of plating solution through the through holes on theprinted circuit board will repetitively reverse direction.
 9. Theplating apparatus of claim 7 wherein said agitation means comprisesrotating paddle wheels in each of said cells.
 10. The plating apparatusof claim 7 for electroplating of through holes in a printed circuitboard further comprisingion replenishment means containing at least onepiece of the metal to be plated within the plating solution forreplenishing the ion content in the plating solution in said firstcells; first electrode means for providing electrical contact between afirst terminal of a power supply and said at least one piece of metal tobe plated; and second electrode means for providing electrical contactbetween a second terminal of a power supply and the printed circuitboard to be plated.
 11. The plating apparatus of claim 10 wherein saidion replenishment means comprises baskets containing pieces of the metalto be plated in third cells coupled to said first cells and throughwhich plating solution passes in flowing to said first cells.
 12. Theplating apparatus of claim 11 further comprised of fourth cells coupledto said second cells, and through which plating solution passes inflowing from said second cells to said pump means, said forth cellscontaining filter means for filtering of plating solution flowingtherethrough.
 13. Plating apparatus for the plating of through holes ina printed circuit board comprisinga container for confining a platingsolution; printed circuit board support means for supporting a printedcircuit board within said container, reciprocating means coupled to saidsupport means for providing repetitive movement of said circuit boardheld in said support means back and forth in a plane substantiallycoincident with the plane of said circuit board; divider means adjacenteach side of said printed circuit board support means for defining firstand second cells along each side of said printed circuit board supportmeans; agitator means within each of said first cells for mechanicallyproviding substantial agitation to a plating solution therein, saidagitator means comprising rotating paddle wheels within each of saidfirst cells; pump means for delivering plating solution to said firstcells and for removing plating solution from said second cells, saidsecond cells being substantially opposite one of said first cells,whereby fresh plating solution will be urged to flow from said firstcells through the through holes in a printed circuit board on saidprinted circuit board support means and into said second cells toprovide fresh plating solution to inner surfaces of the through holes;ion replenishment means containing at least one piece of a metal to beplated within the plating solution for replenishing the ion content inthe plating solution in said first cells, said ion replenishment meanscomprising baskets containing pieces of the metal to be plated in thirdcells coupled to said first cells and through which plating solutionpasses in flowing to said first cells; first electrode means forproviding electrical contact between a first terminal of a power supplyand said at least one piece of metal to be plated; second electrodemeans for providing electrical contact between a second terminal of apower supply and the printed circuit board to be plated; fourth cellscoupled to said second cells, and through which plating solution passesin flowing from said second cells to said pump means, said fourth cellscontaining filter means for filtering of plating solution flowingtherethrough.
 14. The apparatus of claim 13 wherein the flow of platingsolution in adjacent cells is in opposite directions.